THE BEAVER 679 
and flat, its greatest width (about half-way down) equalling about one- 
third of its length; at its base it is densely furred, but elsewhere its 
covering consists of large, flattened scales, between which are scattered 
a few short, stiff hairs. The urino-genital organs, anal glands, and 
anus open into a cloaca, sothat apart from the mamme, of which the 
female has four, the sexes look alike externally. 
The pelage consists chiefly of a very dense, long underfur (about 
25 mm. long on back), with a far scantier growth of long, coarse hairs 
(50 to 60 mm. long); the latter are more abundant on the back, where 
they nearly conceal the underfur, but towards the flanks and on the 
underside they are more scattered, and leave the underfur plainly 
visible. The upper surfaces of the hands and feet are clothed with 
short hair, which completely conceals the scales. 
The general colour of the head and body is a lighter or darker 
yellowish-brown, imparted chiefly by the longer hairs: the tint is 
usually duller below than on the back, and greyer or more pallid about 
the face and chin; the hairy parts of the hands and feet are brown; the 
naked muzzle pad, soles, palms, and the scaly portion of the tail are 
dusky. Partly or wholly albinistic specimens are sometimes met with, 
and their skins have sometimes a beautiful, iridescent lustre. 
In addition to the family peculiarities described above, the strongly- 
built skull is characterised by the remarkable cellular excavation of 
the ventral surface of the basioccipital. The auditory bull are small, 
and each has a very long, tubular or spoutlike, external meatus. The 
zygomatic arches are very strong and widely expanded. The anterior 
palatal foramina are short and narrow; the tooth-rows are anteriorly 
‘convergent, and the pterygoid fosse are deep, the short alisphenoid 
canals opening into their outer margins. The nasals taper posteriorly 
and terminate in the interorbital region well behind the ends of the 
ascending branches of the premaxille; in the American Beavers the 
nasals are shorter and terminate posteriorly at the level of the 
lachrymals. 
The mandible is strong and deep; its rami are very firmly 
connected with each other: the strong coronoid processes rise consider- 
ably above the rounded condyles; the angular processes are rather 
small, and their posterior tips rise above the level of the cheek-teeth. 
Dentition :—Both the incisors and the cheek-teeth when unworn 
show traces of a former, primitive, tubercular or brachyodont structure. 
The tips of quite unworn incisors are very slender, rounded, and 
coated on both their anterior and posterior surfaces with enamel; they 
show more or less distinct traces of apical tubercles and of grooves, 
which appear to represent the valleys which originally separated the 
incisor cusps; such parts are, of course, wholly ephemeral, and as 
vanishing structures betray a high degree of individual variability. 
